Recent advances in understanding the molecular mechanisms of cancer have led to the discovery and development of anticancer therapeutic agents targeting important signaling pathways. These agents typically provide greater therapeutic benefit to the patients with lesser toxicity as compared to the conventional cytotoxic agents. However, patients often face the inevitable reality of recurrence of the cancer due to acquired resistance to the targeted therapeutic agents. There is a great unmet medical need to preempt or address such acquired resistance to cancer therapies.
Two pathways, RAS/RAF/MEK/ERK and PI3K/AKT/PTEN/mTOR, play an important role in the initiation and progression of tumors. Recently, there has been intense activity to discover and develop new agents targeting these two pathways. The RAS/RAF/MEK/ERK pathway is known to be dysregulated through genetic mutations in RAS, RAF or MEK genes, which leads to increased cell proliferation and angiogenesis. These mutations have been found in wide variety tumors. Inhibition of any of these targets was found to effectively inhibit the growth of tumors either in preclinical models or in humans. Recently, several compounds have been discovered which selectively inhibit one of these targets, RAF kinase. The inhibitors of RAF kinase include vemurafenib, dabrafenib, XL-281, LGX-818, CEP-32496 and ARQ-736. Vemurafenib is now an FDA-approved drug for treating the metastatic melanoma patients. Other compounds are in various stages of clinical development, including MEK-162, selumetinib, refametinib, E-6201, pimasertib, WX-554 and GDC-0973. Although several compounds targeting RAS protein have been identified, thus far none of them have been approved by the FDA.
Similar to the RAS pathway, PI3K/AKT/mTOR pathway also plays an important role in tumors, specifically in promoting the tumor cell survival and proliferation. This pathway is dysregulated through genetic changes in PI3K, AKT and PTEN genes. Several proteins in this pathway have been subjected for the drug discovery efforts, leading to the identification of many inhibitors against PI3K, AKT and mTOR proteins, and some of these inhibitors (such as temsirolimus and everolimus) have been approved by the FDA for various indications. Other inhibitors of the PI3K pathway are in various stages of clinical development, including the PI3K inhibitors GDC-0941, PX-866, XL-147, BKM-120, and BAY 80-6946, the mTOR inhibitors deforolimus, OSI-027 and AZD8055, the PI3K/mTOR dual inhibitors BEZ-235, XL-765, GDC-0980, GSK-2126458, PKI-587, and PF-04691502 and the AKT inhibitors MK-206, GDC-0068, GSK2636771, afuresertib, rigosertib and CLR-1401.
Despite some promising initial results in humans, several of the above-mentioned compounds are not able to provide a durable response due to the acquired resistance rendered by the activation of the alternative pathway(s) in the targeted cancer cells. For example, the inhibition of the PI3K pathway with agents such as temsirolimus leads to the subsequent activation of the RAS pathway, resulting in tumors which do not respond to this agent. Conversely, inhibition of RAS pathway leads to the activation of PI3K pathway. Preclinical data has demonstrated that the combinatorial inhibition of both pathways simultaneously gives a greater and more durable efficacy in tumor growth inhibition. These findings clearly indicate a need for combination therapies to overcome the acquired clinical resistance to single-pathway inhibitors. Several clinical trials with different combinations of two agents each inhibiting one of these two pathways have already been initiated. The most advanced combination clinical trial is in Phase II with AZD6244 (a MEK inhibitor) and MK2206 (an AKT inhibitor). However, combining two different agents in this manner can produce the significant disadvantages of added toxicity and higher cost.
Therefore, there is an unmet medical need to identify compounds with dual inhibitory activity against both pathways.